Drop on demand inkjet technology for producing printed images has been employed in products such as printers, multifunction products, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead, onto an image receiving substrate. For example, the image receiving substrate may be moved relative to the printhead and the inkjets may be controlled to emit ink drops through nozzles formed in the printhead at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The ink ejected from the inkjets is liquid ink, such as aqueous, solvent, oil based, curable ink, or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid or a gel form and delivered to a melting device, which heats the ink to generate liquid ink that is supplied to a printhead.
The ejected ink travels through an air gap between the printhead face and the image receiving substrate. The greater the distance between the printhead face and the image receiving member, the greater the force required for the expulsion of the ink to travel this distance and land on the substrate at the position intended for the ejected ink drops. Additionally, a larger air gap enables particulate matter to flow between the printhead face and the substrate. This particulate matter may land on the printhead face and interfere with printhead nozzles or ink drops ejected from the inkjet nozzles.
Inkjet printers that print images on precut sheets of print media are referred to as cut sheet inkjet printers. Cut sheet inkjet printers strip media sheets from a supply of media sheets stacked on an input tray. A media conveyer transports each stripped media sheet through a print zone of the printer. The inkjets eject ink onto the print media as the media conveyer transports the print media through the print zone. After receiving ink from the inkjets, the media conveyer transports the stripped media sheet to an output tray. Once received by the output tray the media sheets are collected by a user or received by another printing system for further processing.
The media conveyer transports the media sheets through the print zone where the printheads are operated to eject ink onto a surface of the media sheets. Accordingly, an air gap is required that is large enough to enable sheets of different thicknesses to pass by the printheads without requiring the inkjet ejectors to expend large amounts of energy to propel the ink drops across the air gap. These competing restrictions on the air gap between the printheads and the media sheets can be balanced provided the media sheets stripped from the input tray are flat and free from creases or other imperfections. Some media sheets stripped from the input tray, however, may include creases and other imperfections. As the media conveyer transports these media sheets, the imperfect portions of the media sheet may pass through the print zone at a distance too close to the printheads for accurate placement of the ink drops. Consequently, image quality may be affected by the close passage of the media sheets to the printhead. For example, some nozzles in the printhead may become clogged by particulate matter carried by a media sheet and image streaks and/or missing pixels may be produced in the printed image. Therefore, control of the distance between media surfaces and the printhead faces in the print zone is useful.